Question 207194
 it takes you 8 hours to dig a hole, you and your brother dig a same size hole and it takes you 5 hours, how long will it take your brother to dig that same hole?
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This type of problem required the sum of reciprocals approach.
I dig 1/8 of the hole per hour.
Bro digs 1/5 of the hole per hour.
Now you can add those.
Together, we dig 1/8 + 1/5 per hour
= 13/40 of the hole per hour.
--> 40/13 hours to dig the hole
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There's a shortcut, which is handy on a timed test, product/sum.
5*8/(5+8) = 40/13